1. Field of the Invention
The present invention relates to an image pickup apparatus and control method therefor, which take a measure for flickers in an arbitrary cycle in an image pickup device and accomplish autofocus for favorable focusing operation.
2. Description of the Related Art
Conventionally, an image pickup apparatus such as a digital video camera or a digital still camera employs a contrast detection method as a method for performing automatic focusing operation using an image signal obtained from an image pickup device such as a CCD.
In the contrast detection method, images are picked up sequentially by moving the position of a focus lens within a predetermined range, a focal point evaluation signal is computed from each image signal, the focusing position is obtained from a focal point evaluation value corresponding to the focal point evaluation signal. A focal point evaluation value is computed so as to be larger as a focal point gets closer to the focusing focal point such as by using a BPF (band-pass filter). Then, the position of the focus lens is controlled toward the position where the focal point evaluation value becomes the maximum so that the focusing state is achieved.
In the contrast detection method, a direction to move a focus lens is obtained from sequential focal point evaluation values. As such, the method has a problem in that favorable focusing operation is not possible if S/N ratios of image signals being the bases of the focal point evaluation values for comparison are largely different from one another. Particularly, it is greatly affected by flickers of fluorescent light.
FIG. 9 is a diagram illustrating an effect of fluorescent flickers when a cycle of readout from an image pickup device is 1/60 sec. and a flicker cycle due to a power supply frequency is 1/50 sec.
In FIG. 9, reference numeral 901 denotes a cycle of readout from an image pickup device, reference numeral 902 denotes accumulation time for the image pickup device, reference numeral 903 denotes brightness changes due to flickers of fluorescent light, reference numeral 904 denotes a cycle of sampling focal point evaluation values, and reference numeral 905 denotes image signals outputted from the image pickup device.
In FIG. 9, the brightness changes due to the flickers of fluorescent light during the accumulation time for the image pickup device differ from one another for respective read out image signals. Accordingly, sequential image signals have different S/N ratios, so that a focal point evaluation value varies even if an object does not change. As a result, favorable focusing operation cannot be performed.
There is a further problem in that while an S/N ratio is steadily low such as at a low-light intensity, a change in an image signal is minor due to a noise effect so that the favorable focusing operation cannot be performed similarly to the brightness changes due to flickers.
To solve the above problems, the following techniques have been proposed.
It is determined whether or not the light intensity is low by comparing a focal point evaluation value gained from an image signal and a comparison value. If the light intensity is sufficient, accumulation time for an image pickup device is modified; and if the light intensity is low, a cycle of sampling focal point evaluation values is modified. This eliminates a flicker effect (for example, see Japanese Patent Laid-Open Patent Publication (Kokai) No. 8-79595).
There has been also proposed a technique capable of eliminating a flicker effect by adjusting a gain of an image signal subjected to wave detection and computation for cancelling a flicker effect by using the computation result (for example, see Japanese Patent Laid-Open Patent Publication (Kokai) No. 2001-186407).
In recent yeas, apart from the above techniques, there has been proposed an image pickup apparatus which reads out image signals at a high speed and stores the signals. FIG. 10 is a diagram illustrating an effect of fluorescent flickers when a cycle of readout from an image pickup device is 1/500 sec. and a flicker cycle due to a power supply frequency is 1/50 sec. during fast readout from the image pickup device. In FIG. 10, reference numeral 1001 denotes a cycle of readout from the image pickup device, reference numeral 1002 denotes accumulation time for the image pickup device, reference numeral 1003 denotes brightness changes due to fluorescent flickers, reference numeral 1004 denotes a cycle of sampling focal point evaluation values, and reference numeral 1005 denotes image signals outputted from the image pickup device.
Such fast readout shortens a cycle of sampling focal point evaluation values, thereby improving autofocus responsiveness.
However, the method of modifying accumulation time for an image pickup device disclosed in the above Japanese Patent Laid-Open Patent Publication (Kokai) No. 8-79595 cannot eliminate a flicker effect if a read-out cycle for the image pickup device exceeds two times of a power supply frequency. The method of modifying a cycle sampling focal point evaluation values also has a problem in that autofocus responsiveness largely decreases.
Additionally, the method of adjusting a gain disclosed in the above Japanese Patent Laid-Open Patent Publication (Kokai) No. 2001-186407 does not improve an S/N ratio. As such, the method has a problem in that an S/N ratio is very low in a flicker valleys indicated by a triangle “▴” in FIG. 10 during fast readout so that favorable focusing operation is not possible.